1. Technical Field
This invention relates to crosshead piston assemblies used in heavy duty diesel engine applications and to the three-piece bearing inserts of such pistons used to support the wrist pins.
2. Related Art
U.S. Pat. No. 4,644,853 discloses a crosshead piston assembly having a three-piece insert bearing used to support the piston pin carried at the end of a connecting rod. The upper half bearing portion is formed with an oil supply hole which is aligned with an oil supply hole formed in the upper saddle portion of the piston for supplying oil issuing from a passage in the piston pin up into an oil cooling space below the head of the piston for cooling the head during use. The bearings are retained against movement by engagement at their edges with retaining shoulder portions of the piston projecting between the facing edges of the upper and lower bearing sets.
U.S. Pat. No. 5,072,654 discloses a similar crosshead piston arrangement, but which dispenses with retaining shoulders in favor of a hollow retaining pin installed in the aligned oil supply holes of the upper bearing and saddle portions or by outwardly turned tab portions formed as a collar extending into the oil supply passage of the saddle portion or as tabs carried at the ends of the upper bearing portion which are received into notches machined in the piston body. The installation of any bearing retaining feature in the oil supply passages of the upper bearing portion and saddle portion, either in the form of inserts or integral collar portions, has the effect of restricting the size of the opening and thus may impede the flow of oil through the passage, reducing the cooling effect that might otherwise be maximized if the passage were free from obstruction of bearing retaining features. The alternative of forming tab portions at the ends of the bearings which fit into machined recesses in the piston adds cost and complexity to the manufacturer of pistons and bearing assemblies.
Accordingly, there is a need in the industry for further alternatives to retaining the bearing sets without obstructing the free flow of cooling oil to the head of the piston and without complicating the manufacture of the bearings and piston body such as by inter fitting tabs and recesses of the bearings and piston body.
According to a first aspect of the invention, a piston assembly is provided having a piston body formed with an upper head portion having a plurality of ring grooves and a lower stem portion extending downwardly from the head portion along a longitudinal axis of the body. The lower stem portion is formed with a transverse pin bore which extends between laterally opposite outer surfaces of the lower stem portion. The lower stem portion also includes a pair of mounting ears having laterally inner surfaces which are spaced from one another across a gap between the ears. The ears present lower concave bearing support surfaces which are likewise interrupted by the gap. The lower stem portion also includes an upper concave bearing support surface which is arranged opposite the lower bearing support surfaces and extends continuously between the laterally outer surfaces of the lower stem portion. The upper bearing support surface is formed with an oil supply hole about midway between the laterally outer surfaces of the lower stem portion. The oil supply hole opens to an oil cooling space for accommodating a flow of cooling oil to the upper head portion of the piston body. A three-piece slipper bearing set is disposed in the pin bore, including a pair of lower portions supported by the lower bearing support surfaces of the ears and a separate upper bearing portion supported by the upper bearing support surface and extending continuously between the laterally outer surfaces of the lower stem portion. The upper bearing portion is formed with an oil supply hole which is aligned with the oil supply hole of the bearing support surface for accommodating the flow of oil. A wrist pin is disposed in the pin bore and is supported by the three-piece slipper bearing set. The wrist pin has an oil supply hole which is aligned with the oil supply holes of the upper bearing portion and upper bearing support surface. The oil supply holes are free and unencumbered by any bearing retention structure which would operate to support the upper bearing portion against movement relative to the upper bearing support surface. The invention is characterized by including aligned retaining holes formed in the upper bearing support surface and the upper bearing portion in spaced relation to the oil supply holes, and a retention pin disposed in the aligned retaining holes to support the upper bearing portion against movement relative to the upper bearing support surface independently of the oil supply holes.
According to a further aspect of the invention, such a three-piece sliding bearing set is contemplated separate from the piston structure having the central oil supply hole formed in the upper section of the bearing set together with a second bearing retention hole formed in the upper section of the bearing set in spaced relation to the oil supply hole.
The piston and bearing assemblies according to the invention have the advantage of providing a simple way of retaining the three-piece bearing set within the pin bore of a crosshead piston without encumbering the oil passage hole used to deliver cooling oil to the head of the piston, and without complicating the design and manufacture of the bearings and piston body through provision of added tab projections on the bearings and recesses machined in the piston body.
The simple solution provided by the invention is to maintain the oil supply holes in their fully opened, unencumbered condition, and to retain the upper bearing section by means of the formation of a set of aligned retention holes in the upper bearing section and its support surface spaced from the oil supply hole and in which a retention pin is disposed for retaining the upper bearing section against movement relative to the support surface without obstructing the central supply passage through the bearing and upper support surface leading to the cooling space beneath the upper head portion of the piston.
Another advantage of the present invention is that it provides a simple solution for securing the three-piece bearing set within a crosshead piston body without complicating the manufacture of either the bearing or head. According to the invention, all that is required is to provide at least one additional opening through the upper bearing portion aligned with an opening formed in the adjacent support surface to receive a simple retainer pin, which may be installed from below in the gap between the mounting ears of the piston body before or after the lower bearing portions are seated. As such, there is no need to provide additional tabs or projection from the bearing parts which would fit into machined recesses in the piston body as proposed by the prior art.